I do think it's possible that someone with depression can live - and suffer - a seriously bad time for a long time, and then suddenly be overwhelmed with a surge of such force that death is the immediate option.
A friend of mine came home from work, had tea, and was due to meet his girlfriend that evening. His mum went next door for fifteen minutes. When she came back, he was not downstairs, so she assumed he was getting ready to go out. When his girlfriend phoned to say he was late, she went upstairs, to find him hanging from a roof beam in the loft.
As with all these terrible situations, no-one could believe that it had happened. I was with him the previous evening, he seemed fine, but as I now know, that is absolutely no indication of a perilous mental state.
I do believe that this sudden onset of self-destruction can occur in minutes, and be acted upon instantly - which does explain apparently 'long term' actions and behaviour right up to the time of death.